A Light Emitting Diode (LED) is a solid state device that converts electrical energy to light. Light is emitted from an active layer of semiconductor materials sandwiched between oppositely doped layers when a voltage is applied across the doped layers. One type of LED device, referred to here as a blue LED device, involves gallium nitride (GaN) layers.
FIGS. 1-3 (Prior Art) illustrate a prior art process used to make such a prior art blue LED device. A buffer layer 50 is grown on a silicon substrate 52, and a template layer 51 is grown on the buffer layer. An epitaxial LED structure 53 including an n-type layer 54, an active layer 55, and a p-type layer 56 is grown on the template layer. A silver mirror layer 57 is formed over the epitaxial LED structure. A silver encapsulation layer 58 is formed over the silver layer 57. A barrier layer 59 is formed over the encapsulation layer 58. The resulting device wafer structure 60 is then wafer bonded to a carrier wafer structure 61 as illustrated in FIG. 1 to form a wafer bonded structure 66.
Carrier wafer structure 61 involves a conductive carrier 63, a sublayer 64 of gold, and a sublayer 65 of gold/tin as illustrated. Prior to wafer bonding, there is also a sublayer 62 of gold on the top of the device wafer structure 60. The gold sublayers and the gold/tin sublayer are together referred to as a wafer bonding layer.
Wafer bonding involves pressing the two wafer structures 60 and 61 together and raising the temperature such that the gold/tin sublayer 65 melts. After cooling, when the gold/tin sublayer resolidifies, the two wafer structures are bonded together to form the wafer bonded structure 66. If during the wafer bonding process any of gold or tin were to diffuse into the silver layer 57, then the reflectivity of the silver might be decreased and/or the resistance of the contact at the silver-to-p-layer interface might be increased. Barrier layer 59 is provided to prevent such unwanted diffusion. FIG. 1 shows the multi-layer structure of the barrier layer 59 in the prior art example of FIGS. 1-3. Barrier layer 59 includes multiple periods. Each period includes a platinum sublayer and a titanium-tungsten sublayer.
Silicon is opaque to the blue light emitted by the active layer. Silicon substrate 52 and buffer layer 50 are therefore removed from the bonded wafer structure, thereby forming the wafer bonded structure 66 illustrated in FIG. 2. The upper surface of structure 66 is then roughened, and an n-type metal electrode 67 is added to the top of the structure, and a p-type metal electrode 68 is added to the bottom of the structure. The wafer bonded structure with its electrodes is then singulated to form individual LED devices.
FIG. 3 (Prior Art) shows one of these LED devices as part of a white LED assembly 74. The LED device is attached to a first portion of metal 69 of a metal core Printed Circuit Board (PCB) 70 or other package in what is commonly referred to as “die attach”. The LED device cannot be heated to too high a temperature during the die attach process or the gold/tin wafer bond material will melt and the LED device will come apart. Silver epoxy 71, that is applied and cures in a temperature environment that does not exceed 150° C., is therefore commonly used. The n-electrode 67 is typically connected via a wire bond 72 to a second portion of metal 73 of the metal core PCB 70 as illustrated in FIG. 3.